Image compression method, decompression method thereof and program therefor

Abstract

In a method of image compression in which vertical and horizontal white skipping are performed, data compression is performed in the regions containing black pixels other than the skipped regions. While vertical and horizontal white skipping being performed, in the other regions, regions are detected in which pixel values change in smooth fashion, such as the outlines of letters and encoding is performed using the pixel value of the leading column of the region, number of columns and amount of change of the pixel values. As a result, the compression factor of the image can be increased and a considerable contribution made to reduce the storage capacity for storage of data and reduce the time required for data transmission.

1. A method of image compression comprising:
 a first region separation step of separating into regions containing black pixels and regions not containing black pixels in units of K rows (where K is an integer of one or more) by scanning an image in a main scanning direction; 
 a second region separating step of separating regions containing black pixels and regions not containing black pixels in single column units by scanning in an auxiliary scanning direction intersecting said main scanning direction in logical rows defined by L (where L is an integer of one or more) rows including at least one row containing black pixels obtained by the first region separating step; 
 a third region separating step of separating regions in which black pixels increase or decrease continuously in the same direction of the auxiliary scanning direction in the logical row in accordance with the separation results of said second region separating step; and 
 an encoding step of encoding by using the regions obtained in said first, second and third region separating step as elements; 
 wherein the encoding step includes a step of encoding the regions separated by said third region separating step with the number of columns of said region, the pixel value of the leading column of the region and the amount of change of the black pixels of each column of the second and subsequent columns of said region with respect to the immediately preceding column. 
 
   
   
     2. The method of image compression according to  claim 1 ,
 wherein said third region separating step comprises a step of separating the regions in which black pixels increase or decrease continuously in said same direction with reference to the relationship of black pixels of adjacent columns of said region containing black pixels. 
 
   
   
     3. The method of image compression according to  claim 1 ,
 wherein said third region separating step comprises a step of separating the regions in which black pixels increase or decrease continuously in said same direction by using the pixel values of adjacent columns to reference a separation criteria table which stores information indicating the subject of detection, the direction of change and the amount of change corresponding to the pixel values of these two columns. 
 
   
   
     4. The method of image compression according to  claim 1 ,
 wherein said encoding step comprises a step of encoding by using a single code indicating the amount of change when the amount of change of black pixels in each column in the region separated by said third region separating step is constant. 
 
   
   
     5. The method of image compression according to  claim 1 ,
 wherein said encoding step comprises a step of expressing the code indicating the amount of change of the number of black pixels of each column by N bits satisfying M≦2 N −1 (where N is an integer of one or more), when the amount of change of the number of black pixels of each column in the region separated by said third region separating step is a maximum M pixels. 
 
   
   
     6. A method of image decompression for decompressing compressed data obtained by separating into regions containing black pixels and regions not containing black pixels in units of K rows (where K is an integer of one or more) by scanning an image in a main scanning direction, separating regions containing black pixels and regions not containing black pixels in single column units by scanning in an auxiliary scanning direction intersecting this main scanning direction in logical rows defined by L (where L is an integer of one or more) rows including at least one row containing black pixels, separating into regions in which black pixels increase or decrease continuously in the same direction of the auxiliary scanning direction in the logical row and encoding by using each the separated regions as elements, comprising the steps of:
 analyzing a code word of the region from the compressed data; and 
 restoring regions in which black pixels increase or decrease continuously in the same direction of the auxiliary scanning direction in the logical row obtained by said analysis, from the number of columns of the region, the pixel values of the leading column of the region and the amount of change of black pixels with respect to the immediately preceding column of the second and subsequent columns of the region contained in the code word. 
 
   
   
     7. A computer-readable medium encoded with a computer program, comprising:
 first region separating program data for separating into regions containing black pixels and regions not containing black pixels in units of K rows (where K is an integer of one or more) by scanning an image in a main scanning direction; 
 second region separating program data for separating regions containing black pixels and regions not containing black pixels in single column units by scanning in an auxiliary scanning direction intersecting the main scanning direction in logical rows defined by L (where L is an integer of one or more) rows including at least one row containing black pixels obtained by the first region separating program data; 
 third region separating program data for separating regions in which black pixels increase or decrease continuously in the same direction of the auxiliary scanning direction in the logical row in accordance with the separation results of said second region separating program data; and 
 an encoding program data for encoding by using the regions obtained in said first, second and third region separating program data as elements; 
 wherein the encoding program data includes program data for encoding the regions separated by said third region separating program data with the number of columns of said region, the pixel value of the leading column of the region and the amount of change of the black pixels of each column of the second and subsequent columns of said region with respect to the immediately preceding column. 
 
   
   
     8. A computer-readable medium encoded with a computer program for decompressing compressed data obtained by separating into regions containing black pixels and regions not containing black pixels in units of K rows (where K is an integer of one or more), by:
 scanning an image in a main scanning direction, separating regions containing black pixels and regions not containing black pixels in single column units by scanning in an auxiliary scanning direction intersecting the main scanning direction in logical rows defined by L (where L is an integer of one or more) rows including at least one row containing black pixels, separating into regions in which black pixels increase or decrease continuously in the same direction of the auxiliary scanning direction in the logical row and encoding by using each the separated regions as elements, comprising: 
 analyzing program data for analyzing a code word of the region from the compressed data; and 
 restoring program data for restoring regions in which black pixels increase or decrease continuously in the same direction of the auxiliary scanning direction in the logical row obtained by said analysis, from the number of columns of the region, the pixel values of the leading column of the region and the amount of change of black pixels with respect to the immediately preceding column of the second and subsequent columns of the region contained in the code word.